1. Field of the Invention
This invention relates generally to apparatus for imparting seismic vibratory signals to the earth. More particularly, this invention relates to improved apparatus for imparting seismic vibratory signals to the earth along a selected vector path.
2. Description of the Prior Art
The development of multi-component seismic sources has enabled the operators more efficiently to measure sub-surface formation properties such as porosity, fracture orientation, pore fluid type or lithologic type. It is well-known that compressional and shear waves travel with distinct velocities when passing through a particular geologic formation. Changes in formation property, such as rock type, porosity and the like may be detected by changes in the full wave field which includes the compressional and shear wave velocities. By recording the full wave form of a seismic wave, including the compressional and shear waves, one can get far more detailed information concerning rock properties at depth.
A method and apparatus which provides simultaneous compressional and shear waves utilizing but a single vibrating mass is taught in U.S. Pat. No. 4,662,473 issued May 5, 1987. The apparatus described in the patent couples a vibrating mass to a ground engaging base plate so as to enable pivotal rotation of the vibrating mass about two mutually perpendicular rotational axes. In this way, the vibratory axis of the vibrating mass may be made to coincide with any pre-selected vector path. This vector path can then be oriented with respect to a remotely located geophone so as to enable simultaneous generation and detection of compressional and shear waves. The relative amplitudes of such waves are varied by selecting any desired combination of inclination and azimuth for the vibratory axis. The vibrating mass is linked to the base plate by way of a ball and socket joint which constitutes a universal pivot for the vibratory axis of the mass. Mechanical orienting means is provided to orient the vibrating mass along any selected vector path, the ball being freely movable within the socket. The vibratory mass is supported in the selected orientation by the orienting mechanism.
The present invention has a general objective of an apparatus which provides for the generation of simultaneous compressional and shear waves by means of vibrational energy.
A more specific objective of this invention is to provide an improved apparatus for the simultaneous generation of compressional and shear waves wherein the orientation of the apparatus along a selected vector path is devoid of any supplemental mechanical orientation means and the vibrating mass or reactor is locked in place relative to the base plate.